Broadleaf plants (also called "broad-leaved") are those with leaves that have a flat, relatively broad surface. This surface is often marked with a network of prominent veins. These botanical characteristics distinguish them from plants with needle-like, awl-like, scale-like, or blade-like leaves. For purposes of categorization, this distinction allows us to group together plants that share the same characteristic.
In general landscaping and gardening activities, the term broadleaf is often used to refer to shrubs and trees that have "typical" leaves, rather than those with leaves shaped like needles, etc. Note that "broadleaf" and "evergreen" aren't necessarily opposites: There are many evergreen plants such as azaleas (Rhododendron), as well as mountain laurels (Kalmia latifolia), that are broadleaf shrubs, despite sharing the "evergreen" classification with needle-bearing shrubs such as yew (Taxus). Meanwhile, there are other evergreen shrubs have awl-like foliage, such as junipers (Juniperus). Still others have scale-like foliage, including arborvitae (Thuja).
"Broadleaf" is also a term used to describe a large group of lawn and garden weeds to distinguish them from grassy, blade-leaved weeds, such as crabgrass and quackgrass.
Broad-Leaved Trees and Shrubs
Some of the following examples are plants with which you have probably been familiar since childhood. The full list includes all the deciduous shrubs and trees whose fall foliage we so eagerly await every autumn:
- Red maple tree (Acer rubrum)
- Japanese maple tree (Acer palmatum, etc.)
- Oak tree (Quercus rubra)
- Tulip tree (Liriodendron tulipifera)
- Poplar tree (Populus nigra)
- Loropetalum shrubs
- Oakleaf hydrangea shrub (Hydrangea quercifolia)
- Bottlebrush shrub (Fothergilla gardenii)
- Burning bush shrub (Euonymus alata)
- Sumac shrub (Rhus glabra, etc.)
Broadleaf Is Not the Same as Deciduous
The term "deciduous" is sometimes used as a synonym for "broadleaf" when it comes to describing trees and shrubs, but strictly speaking, the terms have different meanings. Deciduous derives from the Latin word decidere, meaning "to fall off," and in horticulture, it refers to plants that lose all of their leaves for a portion of the year. Deciduous is another large, general classification use to distinguish plants from other types that do not lose all their leaves—the evergreens.
The deciduous broadleaf trees and shrubs have developed a strategy whereby they enjoy the best of both worlds. During the warm-weather months, the relatively large surface area of their leaves makes them powerful photosynthesis machines, soaking up as much sunshine as possible to convert light energy into chemical energy for growth. Then, when temperatures or moisture levels fall, they shed their leaves and go dormant. This trait is an evolved survival mechanism: The large surface area that is such an advantage for the leaves during the warm weather would become a disadvantage during the cold weather when it allows moisture to escape the plant.
"Deciduous" and "broadleaf" are not, however, synonymous terms in the world of trees. Most, but not all, broadleaf trees and shrubs are deciduous. The live oak (Quercus virginiana) is an example of a broadleaf tree that is evergreen, not deciduous. But this exception should not surprise us since the live oak is a tree of the American South, where winters are relatively mild. There are also some plants with needle-like leaves that are deciduous rather than evergreen. The larch and cypress are examples of conifers that shed their needles each year.
Why Leaves Change Color in Fall
This shedding of the leaves is preceded by the glorious fall foliage stage. We owe the changing leaf color in fall to a subtraction, not to an addition. The tree seals off the leaves from their stems, shutting off their water supply. Thus deprived of water, the leaves cease to make chlorophyll—the substance that makes leaves appear green all summer: The chlorophyll was masking other colors in the leaves. So, in a sense, the breathtaking fall foliage season is the result of an unmasking, in which the leaves' true colors are revealed.
Grassy vs. Broadleaf Weeds
The term "broadleaf" isn't reserved solely for trees and shrubs. The term is also often applied to common lawn weeds fitting that description, as a means of separating them from other weeds—a distinction that is helpful when it comes to controlling through the use of herbicidal sprays. Many beginners fail to realize that how you battle a weed in your lawn very much depends on whether it is broad-leaved or not.
Another group of weeds, the so-called "grassy" weeds, have blade-like leaves and are also frequently found in lawns. Because such weeds are botanically similar to the "good" grass (the lawn grass that you wish to keep), you must use special herbicides on them. Otherwise, you will kill your lawn grass in the process.
An example of a grassy weed is crabgrass (Digitaria), the bane of all lawn enthusiasts. Homeowners spend countless hours researching and using the best crabgrass killers annually. Some also regard tall fescue (Festuca arundinacea) as one of the grassy weeds.
You need to use an entirely different type of herbicide to kill broadleaf weeds. When you are at a home improvement store, look for herbicidal products that are specifically labeled to indicate they are intended for broadleaf weeds. Broadleaf weedkillers include chemicals that are readily absorbed by the leaf structure of broadleaf plants, Herbicides that phenoxy herbicides, such as 2,4-D, mecoprop (MCPP), 2,4-DP (dichlorprop), MCPA and benzoic acid dicamba, offer good broadleaf weed control. Examples of broadleaf weeds commonly found in lawns include:
Although broadleaf plants are distinguished from, say, needle-bearing evergreens by the breadth of their leaves, it's hardly the case that all broadleaf plants have particularly large leaves. For example, boxwood shrubs (Buxus) are broad-leaved, but their leaves are tiny compared to the leaves of big-leaf hydrangeas (Hydrangea macrophylla). Willows (Salix) is another plant considered broad-leaved yet it also bears narrow leaves. Although botanists know exactly what they are talking about when they discuss "broadleaf" plants, the term is not clear-cut as a beginner might hope.